Where can I find the Tsuchinoko?

First, grab a couple of mousetraps in Bolshaya Past Base. There should be one in the food shed and one under the beds in the building.

Next, return to Bolshaya Past South and place the mousetraps in the grass, near to the northernmost electric fence. For best results, put one mousetrap in each of the two grassy patches.

Now, just wait. After awhile, one of the mousetraps will snap shut, catching a live Tsuchinoko for you. If you listen closely enough, you should hear the 'item appear' sound effect when the trap closes, no matter where you are in the area.

Keep it ALIVE until the torture scene, where you lose the Tsuchinoko. After you escape from Groznyj Grad and return to the waterfall where EVA gives you your gear back, go back outside of the waterfall and look under the nearest tree. You should find the Tsuchinoko slithering about; simply tranquilize it with the Mk22 or EZ Gun and collect it.

Remember, in order to claim the rewards associated with the Tsuchinoko, it has to be kept alive until the very end of the game.